The present invention relates to wafer polishing apparatus in general and to measuring systems incorporated into such apparatus in particular.
Wafer polishing systems are known in the art. They polish the top layer of semiconductor wafers to a desired thickness. To do so, the wafer being polished is immersed in a slurry of water and chemicals during the polishing process. Once the wafer has been polished and washed down, it is placed into an exit station known by some companies as a xe2x80x9cwater trackxe2x80x9d, after which the wafer is placed into a cassette of wafers. The cassette is maintained within a water bath until full, after which the entire cassette is brought to a cleaning station to remove any chemicals and slurry particles still remaining on the wafers in the cassette and to dry the wafers. After cleaning, the wafers are brought to a measurement station to determine if the polisher produced the desired thickness of their top layers.
FIG. 1, to which reference is now briefly made, illustrates a prior art water track, such as the water track of the #372 Polisher manufactured by IPEC Westech Inc. of Phoenix, Ariz., USA. The water track, labeled 10, comprises a frame 12 and a base 14. Frame 12 has jet holes 16 connected to jets (not shown) which emit streams 18 of water through holes 16. Base 14 has holes 20 connected to bubblers (not shown) which bubble small amounts of water 22 through holes 20. When a wafer 25 is dropped into water track 10, pattern-side down, the jets and bubblers are activated. Streams 18, from the water jets, serve to force the wafer 25 in the direction indicated by arrow 24. Small streams 22 push the wafer 25 slightly away from the base 14 and ensure that, while the wafer 25 moves through the track, it never rubs against base 14 and thus, the pattern on the wafer is not scratched.
Other companies produce polishers whose exit stations are formed just of the cassettes. Such a polisher is the 6DS-SP polisher of R. Howard Strasbaugh Inc. San luis Obispo, Calif., USA.
It is an object of the present invention to provide a measurement system installable within a polishing machine and, more specifically, within the exit station of a polishing machine.
In accordance with a preferred embodiment of the present invention, the present invention includes an optical system, which views the wafer through a window in the exit station, and a gripping system, which places the wafer in a predetermined viewing location within the exit station while maintaining the patterned surface completely under water. The present invention also includes a pull-down unit for pulling the measurement system slightly below the horizontal prior to the measurement and returns the measuring system to horizontal afterwards.
In accordance with a first preferred embodiment of the present invention, the gripping system includes a raisable gate which collects the wafer in a predetermined location, and a gripper which grips the wafer, carries it to the viewing location and immerses the wafer, along a small angle to the horizontal, in the water. The gripper also holds the wafer in place during the measurement operation, after which, it releases the wafer and the raisable gate is raised.
The present invention incorporates the method of immersing an object into water such that very few bubbles are produced on the wafer surface. The method of the present invention preferably includes the step of immersing the object while it is held such that its surface plane is at a small angle to the horizontal.
In a second embodiment, the measurement system includes a water bath and a gripping system thereabove. The gripping system includes wafer holding elements, which receive the wafer, and a gripper whose initial location is above the expected reception location of the wafer. The gripper is flexibly connected at an angle to a piston such that the wafer is immersed in the water at an angle to the horizontal.